6 PEEFACB 



I wish to express my very great obligations to Professor L. T. 

 Hobhouse, who read through the whole book in manuscript and 

 furnished me with numerous most valuable suggestions and 

 criticisms. To Mr. Julian Huxley, who read the chapters which 

 deal most directly with biological problems, I am also indebted 

 for valuable help. The calculations which appear in the last 

 section of the fifth chapter I owe to Mr. H. T. Tizard. 



